prEN 12255-6

Wastewater treatment plants - Part 6: Activated sludge process

This document specifies performance requirements for treatment of wastewater using the activated sludge process for plants over 50 PT.
A variety of activated sludge systems has been developed. This document has not attempted to specify all available systems. This document provides fundamental information about single stage systems.
The informative Annexes A, B and C provide design information.

Kläranlagen - Teil 6: Belebungsverfahren

Dieses Dokument legt Leistungsanforderungen an die Abwasserreinigung mit dem Belebungsverfahren in Kläranlagen für über 50 EW fest.
Es wurde mittlerweile eine Vielzahl an unterschiedlichen Belebungssystemen entwickelt. Dieses Dokument trifft jedoch keine Festlegungen für alle verfügbaren System. Es macht grundsätzliche Angaben zu einstufigen Anlagen.
Der informative Anhang A, der informative Anhang B und der informative Anhang C enthalten Angaben zur Planung und Auslegung.

Stations d'épuration - Partie 6: Procédé à boues activées

Le présent document spécifie les exigences de performance relatives au traitement des eaux usées à l’aide
du procédé à boues activées pour les stations d’épuration de plus de 50 équivalents-habitants (EH).
Il existe de nombreux systèmes à boues activées. L’objectif du présent document n’est pas de spécifier
tous les systèmes disponibles. Il fournit des informations fondamentales en ce qui concerne les systèmes
mono-étages.
Les Annexes A, B et C, informatives, fournissent des informations de conception.

Čistilne naprave za odpadno vodo – 6. del: Postopek z aktivnim blatom

General Information

• Status: Not Published
• ICS: 13.060.30 - Sewage water
• Technical Committee: CEN/TC 165 - Waste water engineering
• Drafting Committee: CEN/TC 165/WG 40 - Wastewater treatment plants > 50 PT
• Current Stage: 4599 - Dispatch of FV draft to CMC - Finalization for Vote
• Directive: 93/38/EEC - Procurement procedures of entities operating in the water, energy, transport and telecommunications sectors
• Mandate: M/BC/CEN/88/15 - Standardization request to CEN/CENELEC in the field of Water supply, drainage and sewerage
• Ref Project: oSIST prEN 12255-6:2021 - Wastewater treatment plants - Part 6: Activated sludge process

RELATIONS

• Revised: EN 12255-6:2002 - Wastewater treatment plants - Part 6: Activated sludge process
• Effective Date: 04-Apr-2018

Standards Content (sample)

SLOVENSKI STANDARD
oSIST prEN 12255-6:2021
01-september-2021
Čistilne naprave za odpadno vodo – 6. del: Postopek z aktivnim blatom
Wastewater treatment plants - Part 6: Activated sludge process
Kläranlagen - Teil 6: Belebungsverfahren
Stations d'épuration - Partie 6: Procédé à boues activées
Ta slovenski standard je istoveten z: prEN 12255-6
ICS:
13.060.30 Odpadna voda Sewage water
oSIST prEN 12255-6:2021 en,fr,de

2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

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oSIST prEN 12255-6:2021
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oSIST prEN 12255-6:2021
DRAFT
EUROPEAN STANDARD
prEN 12255-6
NORME EUROPÉENNE
EUROPÄISCHE NORM
June 2021
ICS 13.060.30 Will supersede EN 12255-6:2002
English Version
Wastewater treatment plants - Part 6: Activated sludge
process

Stations d'épuration - Partie 6: Procédé à boues Kläranlagen - Teil 6: Belebungsverfahren

activées

This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee

CEN/TC 165.

If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations

which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other

language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC

Management Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,

Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,

Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Turkey and

United Kingdom.

Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are

aware and to provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without

notice and shall not be referred to as a European Standard.
EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION
EUROPÄISCHES KOMITEE FÜR NORMUNG
CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels

© 2021 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 12255-6:2021 E

worldwide for CEN national Members.
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Contents Page

European foreword ...................................................................................................................................................... 4

Introduction .................................................................................................................................................................... 5

1 Scope .................................................................................................................................................................... 6

2 Normative references .................................................................................................................................... 6

3 Terms and definitions ................................................................................................................................... 6

4 Symbols and abbreviations ......................................................................................................................... 7

4.1 Symbols .............................................................................................................................................................. 7

4.2 Indices ................................................................................................................................................................. 8

4.3 Abbreviations ................................................................................................................................................. 10

5 Requirements ................................................................................................................................................. 11

5.1 General.............................................................................................................................................................. 11

5.2 Planning ........................................................................................................................................................... 11

5.2.1 Basic information.......................................................................................................................................... 11

5.2.2 System selection ............................................................................................................................................ 12

5.2.3 Biological reactors ........................................................................................................................................ 17

5.2.4 Clarifiers ........................................................................................................................................................... 17

5.2.5 Environmental impact ................................................................................................................................ 18

5.3 Detailed Design .............................................................................................................................................. 18

5.3.1 Flow-splitting ................................................................................................................................................. 18

5.3.2 Biological reactors ........................................................................................................................................ 18

5.3.3 Mixing ................................................................................................................................................................ 19

5.3.4 Aeration ............................................................................................................................................................ 20

5.3.5 Secondary clarifiers ..................................................................................................................................... 25

5.3.6 Return and surplus sludge systems ....................................................................................................... 27

5.3.7 Internal recirculation .................................................................................................................................. 27

5.3.8 Control and automation ............................................................................................................................. 28

6 Test methods .................................................................................................................................................. 29

Annex A (informative) Design of biological reactors .................................................................................... 30

Annex B (informative) Raw Wastewater Characteristics ............................................................................ 31

Annex C (informative) Removal Efficiency of primary clarifiers .............................................................. 32

Annex D (informative) External Carbon Sources ........................................................................................... 33

Annex E (informative) Sludge age MSRT and aerobic sludge age MASRT ............................................. 34

Annex F (informative) Surplus Sludge production ........................................................................................ 36

Annex G (informative) Denitrification Capacity ............................................................................................. 38

Annex H (informative) Oxygen Consumption .................................................................................................. 39

Annex I (informative) Iterative calculation of the volumetric ratio of denitrification

reactors (V /V) ............................................................................................................................................... 41

Annex J (informative) Reactor Volume (V) ....................................................................................................... 42

Annex K (informative) Internal recirculation ratio (IRR) ........................................................................... 43

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Annex L (informative) Alkalinity ......................................................................................................................... 44

Annex M (informative) Aerobic selectors ......................................................................................................... 46

Annex N (informative) Design based on F/M-ratio ....................................................................................... 47

Annex O (informative) Sludge volume index (SVI) ........................................................................................ 48

Annex P (informative) Solids concentration of the return sludge (C ) ............................................. 49

SS,RS

Annex Q (informative) Return sludge flow (Q ) and solids concentration in the biological

reactor (C ) .................................................................................................................................................. 50

SS,R

Annex R (informative) Surface area (A ) of clarifiers ................................................................................... 52

Annex S (informative) Depth (hC) of clarifiers ................................................................................................ 53

Annex T (informative) Scraper Design............................................................................................................... 54

Annex U (informative) Return sludge balance ................................................................................................ 56

Annex V (informative) Influent structures ....................................................................................................... 57

Annex W (informative) Design of a fine bubble aeration system ............................................................. 58

Bibliography ................................................................................................................................................................. 61

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European foreword

This document (prEN 12255-6:2021) has been prepared by Technical Committee CEN/TC 165 “Waste

water Engineering”, the secretariat of which is held by DIN.
This document is currently submitted to the CEN-Enquiry.
This document will supersede EN 12255-6:2002.

This document has been prepared under a mandate given to CEN by the European Commission and the

European Free Trade Association, and supports essential requirements of EU Directive(s).

This is the sixth part prepared by Working Group CEN/TC 165/WG 40, relating to the general

requirements and processes for treatment plants for a total number of inhabitants and population

equivalents (PT) over 50.

The EN 12255 series with the generic title “Wastewater treatment plants” consists of the following

Parts:
• Part 1: General construction principles
• Part 2: Storm management systems
• Part 3: Preliminary treatment
• Part 4: Primary settlement
• Part 5: Lagooning processes
• Part 6: Activated sludge process
• Part 7: Biological fixed-film reactors
• Part 8: Sludge treatment and storage
• Part 9: Odour control and ventilation
• Part 10: Safety principles
• Part 11: General data required
• Part 12: Control and automation

• Part 13: Chemical treatment — Treatment of wastewater by precipitation/flocculation

• Part 14: Disinfection

• Part 15: Measurement of the oxygen transfer in clean water in aeration tanks of activated sludge

plants
• Part 16: Physical (mechanical) filtration
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Introduction

Differences in wastewater treatment throughout Europe have led to a variety of systems being

developed. This document gives fundamental information about the systems; this document has not

attempted to specify all available systems. A generic arrangement of wastewater treatment plants is

illustrated below:
Key
1 preliminary treatment
2 primary treatment
3 secondary treatment
4 tertiary treatment
5 additional treatment (e.g. disinfection or removal of micropollutants)
6 sludge treatment
7 lagoons (as an alternative)
A raw wastewater
B effluent for re-use (e.g. irrigation)
C discharged effluent
D screenings and grit
E primary sludge
F secondary sludge
G tertiary sludge
H digested sludge
I digester gas
J returned water from dewatering
Figure 1 — Schematic diagram of wastewater treatment plants

The primary application is for wastewater treatment plants designed for the treatment of domestic and

municipal wastewater.

NOTE For requirements on pumping installations at wastewater treatment plants see EN 752 “Drain and

sewer systems outside buildings” and EN 16932, “Drain and sewer systems outside buildings — Pumping

systems”:
— Part 1: General requirements;
— Part 2: Positive pressure systems;
— Part 3: Vacuum systems.
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1 Scope

This document specifies performance requirements for treatment of wastewater using the activated

sludge process for plants over 50 PT.

A variety of activated sludge systems has been developed. This document has not attempted to specify

all available systems. This document provides fundamental information about single stage systems.

The informative Annexes A, B and C provide design information.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

EN 16323, Glossary of wastewater engineering terms

EN 12255-1, Wastewater treatment plants — Part 1: General construction principles

EN 12255-10, Wastewater treatment plants — Part 10: Safety principles
EN 12255-11, Wastewater treatment plants — Part 11: General data required
EN 12255-12, Wastewater treatment plants — Part 12: Control and automation
3 Terms and definitions

For the purposes of this document, the terms and definitions given in EN 16323 and the following apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

• IEC Electropedia: available at https://www.electropedia.org/
• ISO Online browsing platform: available at https://www.iso.org/obp
3.1
enhanced biological phosphorus removal

activated sludge system for increased biological phosphorus removal by luxury uptake whereby mixed

liquor or return sludge is intermittently subjected to anaerobic and aerobic conditions

3.2
internal recirculation ratio
IRR

ratio of the flow of recirculated nitrate containing wastewater to a denitrification reactor relative to the

inflow
3.3
selector

first, small reactor of an activated sludge system where incoming wastewater and return activated

sludge are blended and mixed to subject the return activated sludge to a high sludge load in order to

mitigate sludge bulking

Note 1 to entry: A selector can be aerobic or anaerobic; aerobic selectors are more common. An anaerobic

selector can also be used to assist biological phosphorus removal.
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3.4
mixed liquor suspended solids
MLSS
dry mass concentration of suspended solids in a mixed liquor

Note 1 to entry: the dry mass of filtered solids are determined in accordance with the 23rd, edition of Standard

Methods for Wastewater (SMEWW), 2540 parts D & E.
[SOURCE: EN 16323:2014, definition 2.3.10.24]
3.5
mixed liquor volatile suspended solids
MLVSS
dry mass concentration of organic suspended solids in a mixed liquor

Note 1 to entry: the dry mass of filtered solids are determined in accordance with the 23rd, edition of Standard

Methods for Wastewater (SMEWW), 2540 parts D & E.
[SOURCE: EN 16323:2014, definition 2.3.10.25]
4 Symbols and abbreviations
4.1 Symbols
Symbol Definition Unit
A area m
C mass concentration mg/l
D diameter m
F factor (dimensionless)
F/M load to mass ratio, (e.g. kg BOD5/d per kg MLSS) kg/(kg∙d)
HRT hydraulic retention time (= V/Q) d

IRR - internal recirculation ratio (for recirculation of nitrate) (dimensionless)

L length m
M mass kg
MSRT mean solids retention time = sludge age d or h
OC oxygen consumption kg/h
OTE oxygen transfer efficiency at operational conditions kg/kWh
P power W or kW
Q specific flow per x, e.g. per person m /(h∙x)
Q flow m /h or l/s

RSR return sludge ratio = return sludge flow to wastewater inflow (dimensionless)

S dissolved mass concentration mg/l
SOTR standard oxygen transfer rate kg/h
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Symbol Definition Unit
SSOTR specific standard oxygen transfer rate g/(Nm ∙h)
SOTE standard oxygen transfer efficiency kg/kWh
SSOTE specific standard oxygen transfer efficiency %/m
SSP surplus sludge production kg/d
SVI sludge volume index ml/g
SSVI stirred sludge volume index ml/g
T temperature °C or K
V volume m
W width m
X solid mass concentration mg/l
Y yield kg/kg
a number of scraper arms —
c concentration mmol/l
h height or depth m
l specific load per person or total population and day g/(P∙d)
n number —
p pressure kPa
t time h or s
v velocity m/s
alpha factor = ratio of oxygen transfer coefficients in wastewater to
α —
test water
β salinity factor of test water —
Δp pressure loss kPa
4.2 Indices
A area
Air air
Al aluminium
Al(III) trivalent aluminium (Al )
AR aerobic reactor
BioP enhanced biological P removal
B bottom
Bl blower
BM biomass
BOD biochemical oxygen demand in 5 days
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C carbon
CH methane
Cl clarifier
CO carbon dioxide
COD chemical oxygen demand
D depth
Den denitrification
Dif diffuser
Dos dosing
DS dried solids
EPDM ethylene-propylen-dien class M; a synthetic rubber material
Fe iron
Fe(II) bivalent iron (Fe )
Fe(III) trivalent iron (Fe )
F floor
MAP magnesium-ammonium-phosphate, also called struvite
MLSS mixed liquor suspended solids
MLVSS mixed liquor volatile suspended solids
N nitrogen
Nitr nitrification
NH ammonium
NO nitrate
N2O nitrous oxide or laughing gas
O2 oxygen
OTC oxygen transfer concentration
OT oxygen transfer
P phosphorus
PE-HD polyethylene with high density
PL pipeline
PostD post-denitrification
PreD pre-denitrification
PP polypropylene
PT total population = population + population equivalents
PVC polyvinylchloride
R reactor
RS return sludge
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SA solids per area
SBR sequencing batch reactor
S sludge
SC shortcut
Scr scraper
SimD simultaneous denitrification
SR sludge return
SS suspended solids
St standard
T temperature dependent
TDS total dissolved salt
TKN total Kjeldahl nitrogen
TW test water
WW wastewater
WWTP wastewater treatment plant
4.3 Abbreviations
alk alkalinity
amb ambient
bw backward
cy cycle
dis dissolved
deg degradable or degraded
des design
fw forward
geo geodetic
hor horizontal
h hourly
im immersion
in inflow
inert not degradable
inorg inorganic
int intermittent
max maximum
min minimum
org organic or volatile
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orgN organic nitrogen
out outflow
p particulate
prec precipitated

redeg readily degradable without the need for prior hydrolysis, available for denitrification

sal salinity
sat saturation
th thickening
tot total
vert vertical
20 20 °C
5 Requirements
5.1 General

Biological reactors and final clarifiers are connected by return sludge recirculation lines and form a unit

process: the activated sludge process. The performance of the process depends on biological and

chemical reactions in the activated sludge tanks as well as separation of activated sludge in the final

clarifiers. Activated sludge systems include structures, such as aeration basins and sedimentation tanks,

and technical equipment, such as aeration systems and sludge scrapers.

Biological treatment and clarification (decanting) may be combined in a single sequencing batch reactor

(SBR) with intermittent aeration and sedimentation.

The design shall take account of the requirements specified in EN 12255-1, EN 12255-10, EN 12255-11

and EN 12255-12.
5.2 Planning
5.2.1 Basic information

The design of an activated sludge system shall be based on at least the following information:

1. Maximum and minimum wastewater temperature and temperature-dependent requirements on

the effluent quality;

2. Maximum, minimum and yearly average wastewater inflow; and the maximum 2 h-inflow during

dry weather conditions;

3. System loads, depending on primary treatment (where provided), including variations of COD (or

BOD), SS, P and TKN concentrations. The 85 %-quantiles should be provided for system design and

the 50 %-quantiles (i.e. medians) or arithmetic averages should be provided for the calculation of

operating costs and the design of sludge treatment facilities;

4. Where possible, the composition of the incoming COD shall be provided to the designer, separated

into degradable dissolved COD, inert dissolved COD, degradable particulate COD, inert particulate

COD and readily degradable COD;

NOTE With the standard methods, COD is analysed using dichromate as the oxidising agent. Chrome is a

heavy metal. It would be more sustainable if dichromate could be replaced with a different oxidising agent.

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5. A minimum of 40 samples shall be analysed for all parameters;
6. The consent standards concerning COD, N and P concentrations in the effluent.

Return loads from sludge treatment shall be taken into account, particularly ammonium return load. In

some cases, it may be necessary to provide separate treatment of filtrate or centrifugate from sludge

dewatering, e.g. using a de-ammonification process.

Load removal ratios during primary treatment shall be taken into account. It is recommended to

investigate the removal ratios during dry weather conditions. Where this is not feasible, removal ratios

as shown in Annex C may be used.

Biological treatment units should be protected from excessive hydraulic loads e.g. by the use of

overflow devices and/or storm tanks to meet the required discharge consent. The frequency and

volume of wastewater discharges should be limited (see EN 752).

It is recommended that a half-technical pilot test is performed for a minimum period of half a year

(including the cold weather period) to investigate data for the system design. A design based on long-

term testing can optimize the design and avoid safety factors necessarily included in a more general

design.

Where this is not feasible, Annex A provides basic guidance information for system design.

The following factors shall be determined during planning of an activated sludge system:

• capacity and dimensions of the biological reactors;
• prevention of dead zones and of detrimental deposition in tanks/channels;

• establishment of multiple lines/units or other technical means to maintain the required final

effluent quality while maintenance or repair work is carried out;

• aeration and/or mixing equipment in the biological reactors with sufficient capacity;

• surface area, volume and depth of final clarifiers;
• sludge removal system within clarifiers;
• sludge recirculation and surplus sludge wasting equipment;
• internal recirculation ratio and equipment;
• sufficient stabilization of the removed surplus sludge (where required);
• measurement and control systems;
• odour control;
• noise and vibration control;
• hydraulic head loss.
5.2.2 System selection

The configuration, number, shape and volume of reactors achieving the main biological reactions can

vary considerably according to:
• plant size;
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• the quality of treatment to be achieved, e.g. only BOD removal, nitrification, denitrification and/or

phosphorus removal;

• the requirement for simultaneous aerobic sludge stabilization (i.e. the required aerobic sludge age);

• selection of a single-stage or multi-stage system;

• where biological nitrogen removal is required: selection of the type of denitrification (e.g. pre-,

cascade-, simultaneous, alternative, intermittent or post-denitrification);
• provision of anaerobic or aerobic selectors to mitigate sludge bulking;

• provision of anaerobic reactors to achieve enhanced biological phosphorus removal;

• provision of reactors which can use anoxic or aerobic treatment (depending on load and

temperature);

• requirement for chemical phosphate removal by addition of metal salts (e.g. of ferric, ferrous or

aluminium salts);

• minimum and maximum temperatures, and temperature dependent requirements (e.g. N-removal

requirements).

Where biological nitrogen removal is required, nitrification and denitrification reactors shall be

provided. Six systems can be distinguished (see Figure 1):

1. pre-denitrification in one or several anoxic reactors which are (usually) not aerated;

2. cascade denitrification with alternating anoxic and aerobic reactors whereby the inflow is fed to

anoxic reactors;

3. simultaneous denitrification in a loop reactor (oxidation ditch) with alternating aerobic and anoxic

zones;

4. alternating denitrification with parallel reactors that are sequentially aerated and non-aerated,

whereby the inflow is always fed into the non-aerated reactor;

5. intermittent aeration providing for a sequence of aerobic and anoxic conditions within a reactor,

e.g. in an SBR-reactor; intermittent aeration requires a substantially higher capacity of the aeration

system;

6. post-denitrification with a carbon source fed into the anoxic reactor, followed by a post-aeration

reactor (this system may be used where the C/N-ratio in the influent is so low that a carbon source

shall be added anyway).
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Pre-denitrification and cascade denitrification require recycling of nitrate containing wastewater from

nitrification to denitrification reactors or zones. The internal recirculation ratio depends on the

required denitrification ratio, i.e. the ratio of the mass of nitrogen to be denitrified to the mass of

nitrogen generated, which is dependent on the nitrogen emission consent.

Enhanced biological phosphorus removal may be provided. It may offer the following advantages:

• saving of precipitants;
• reduced dry mass of surplus sludge;
• improved possibility of phosphorus recycling;

• lower reduction of the wastewater’s alkalinity depending on the precipitant used;

• lower concentration of anions (e.g. chloride) in the effluent.
Favourable conditions for enhanced biological P-reduction are:
• high ratio of readily degradable COD to the P-content in the influent;

• low oxygen and nitrate concentration in the flows entering the anaerobic reactor;

• if the flow pattern of the anaerobic reactor is close to a plug flow reactor or where it is a cascade

reactor.

Disadvantages of enhanced Bio-P removal at plants with anaerobic sludge digestion are:

• Sometimes severe precipitation of struvite (or MAP = magnesium-ammonium-phosphate) in

anaerobic digesters and related equipment;

• Dissolved phosphate binds water, reduced the effectiveness of flocculants and impairs the

dewatering results.

The addition of precipitants for P-removal is usually required. For this reason, the capability to add

precipitant dosing facilities shall always be provided for even where they are not initially included.

Selection and design of the activated sludge system may be done with the help of dynamic modelling.

This can be particularly helpful for the upgrading of existing systems.

Figure 2 to Figure 7 show process options for nitrogen removal (nitrification plus denitrification)

systems (Source: DWA-A 131).
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Key
1 denitrification
...